They can be REAL tough to get out. The ring is supposed to compress into the groove as the stub is pulled out. I've seen all kinds of attempts to get them out. A slide hammer hooked to the cv cup seems to be the best, as you mention. You can lay the trans over and soak the area in penetrating oil to loosen the rust. You can apply heat, carefully, just don't cook the seal. Tap the cup to try a break the rust loose. Try both sides, because if you can get one out, you can drive the other out.
I've heard of drilling a hole through one of the stuck stubs to push a rod into to push the other side out, although I've never seen it done. Proceed at your own risk with this method.